A catalyst is a substance that can accelerate the rate of certain chemical reactions, but without being consumed or undergoing a chemical change itself.
Chemical reactions that occur with the evolution of heat to the surroundings are called exothermic reactions, and have a negative enthalpy change, i.e., ΔH<0. Chemical reactions that occur with the absorption of heat from the surroundings are called endothermic reactions, and have a positive enthalpy change, i.e., ΔH>0.